1. Field of the Invention
The present invention generally relates to cathode ray tubes and, more particularly, to implosion protected cathode ray tubes.
2. Description of the Prior Art
A cathode ray tube comprises a glass envelope having an electron gun and a phosphor screen positioned inside and at opposite ends of the glass envelope. In the case of the cathode ray tube used in a television receiver set, the glass envelope is typically of a shape having a generally cylindrical neck portion closed at one end thereof by a stem and accommodating therein the electron gun, a generally funnel-shaped portion flared outwardly from the other end of the neck portion with its opening closed by a faceplate on the inside of which the phosphor screen is deposited.
As a matter of design, the glass envelope, also known as the vacuum enclosure, is highly evacuated to a substantial vacuum and is susceptible to implosion when exposed to severe conditions, for example, abrupt change in temperature or violent handling. The implosion is known as a phenomenon in which the glass envelope collapses inwardly in the presence of a great difference in pressure inside and outside the glass envelope.
The implosion of the glass envelope of the cathode ray tube is accompanied by outward scattering of fragments of glass used to form the glass envelope. The scattering of glass fragments outwardly of the television receiver set is hazardous to television viewers who will be eventually injured when seated close to the television reciever set.
To minimize the hazardous conditions, numerous attempts have hitherto been suggested to provide an implosion protected cathode ray tube, that is, a cathode ray tube designed to have a minimized possibility of outward scattering of glass fragments even when imploded, such as disclosed in, for example, the Japanese Examined Patent Publication (Koho) No. 51-18311 published in 1976; the Japanese Laid-open Patent Publications (Kokais) No. 54-128265 published Oct. 4, 1979, and corresponding to U.S. patent application Ser. No. 890,612 filed Mar. 20, 1978, now U.S. Pat. No. 4,204,231 ; No. 60-47352 published in 1985; and No. 61-124039 published June 11, 1986, and the Japanese Laid-open Utility Model Publications No. 59-71559 published in 1984; No. 59-71560 published in 1984; and No. 59-76065. All of the prior art publications except for the Japanese Examined Patent Publication No. 51-18311 disclose the use of a protective panel fitted to the faceplate of the cathode ray tube in a respective method.
Of these prior art publications, the Japanese Examined Patent Publication No. 51-18311 discloses the use of a reinforcement band of steel encompassed exteriorly around the perimeter of the faceplate of the cathode ray tube. The technique disclosed in this prior art publication is merely to physically reinforce the cathode ray tube, rather than to provide an implosion protected cathode ray tube, and therefore, the cathode ray tube according to this prior art publication still has a problem in that, once the cathode ray tube is imploded, fragments of glass may scatter outwardly of the television receiver set.
The reinforcement of the cathode ray tube by the use of the steel reinforcement band may be satisfactory where the cathode ray tube is of a small size. However, when it comes to the cathode ray tube of about 30 inches or more in screen size, the pressure difference inside and outside the glass envelope is very high and, for example, it is generally recognized that the cathode ray tube of 37 inch screen size is loaded 13 tons due to the high pressure difference. Therefore, the mere use of the steel reinforcement steel band is not an effective measure to the large size cathode ray tube.
The Japanese Laid-open Patent Publication 54-128265 discloses a cathode ray tube wherein a protective panel made of glass material is secured to the faceplate of the glass envelope and spaced therefrom a predetermined distance, for example, 1.6 to 6.4 mm. The space represented by the predetermined distance between the faceplate and the protective panel is filled with thermosetting resin such as polyester resin, polyurethane resin or epoxy resin which, when cured, serves as a bonding agent. To define the space between the faceplate and the protective panel and also to temporarily secure the protective panel to the faceplate during the filling of the thermsetting resin in fluid state into the space between the faceplate and the protective panel, a double sided adhesive tape of generally rectangualr frame structure complemental in shape to the shape of either the faceplate or the protective panel is interposed between the faceplate and the protective panel.
FIG. 4 of the accompanying drawings illustrates another well known version of the cathode ray tube. The glass envelope is identified by 1 and has the faceplate 2 to which a protective panel 7 made of glass material is secured through the intervention of a deposit of bonding material 8, for example, polyurethane resin. The faceplate 2 makes use of a reinforcement band of steel encompassed exteriorly therearound.
While all of the methods disclosed in the prior art publications except for the first-mentioned publication and including the method disclosed in FIG. 4 of the accompanying drawings are generally satisfactory in that the fragments of glass forming the glass envelope will not scatter outwardly of the television receiver set even when the cathode ray tube implodes. However, the use of the protective panel frontwardly of the faceplate has some problems which will now be discussed.
The protective glass panel generally used has a thickness within the range of 3 to 5 mm and therefore has a substantial weight which in turn results in increase of the overall weight of the cathode ray tube. In addition, it is not easy to manufacture the protective glass panel having a surface curvature complemental to that of the faceplate and if not impossible, the protective glass panel requires a high manufacturing cost which eventually results in increase of the manufacturing cost of the cathode ray tube.
Where the protective glass panel having a surface curvature less complemental to that of the faceplate is used and secured to the faceplate with the use of the deposit of bonding material, not only is a substantial amount of bonding material required to secure the protective panel to the faceplate of the cathode ray tube, but also the bond deposit would have a varying thickness between the faceplate and the protective glass panel such that lens-like portions would be formed somewhere in the screen of the cathode ray tube enough to distort corresponding portions of the image being reproduced on the screen.
Moreover, since the protective glass panel is rigid and is not deformable, the protective glass panel which has once been bonded to the faceplate in the wrong way and which therefore requires a re-mounting can hardly be removed from the faceplate in readiness for the re-mounting.
These problems might have been satisfactorily removed according to the Japanese Laid-open Patent Publication No. 61-124039. This prior art publication discloses a cathode ray tube wherein a portion of the glass envelope except for the neck portion is encompassed by a protective covering of heat-shrinkable sheet material together with the reinforcement band encircling exteriorly of the faceplate. However, even the technique disclosed in this prior art publication has a problem in that there is a possibility that, upon the heat shrinkage of the protective sheet, the resultant protective covering may have wrinkles and/or blisters formed therein. Therefore, the use of the heat-shrinkable sheet for the formation of the protective covering appears not to have a favorable productivity.